To close the carbon loop of biomass waste valorization, it is imperative to utilize the unavoidable by-products such as humins, a carbonaceous residue with complex and heterogeneous composition. In this study, starch-rich rice waste was effectively converted into value-added chemicals (e.g., 5-hydroxymethylfurfural) under microwave heating at 160 °C using AlCl3 as the catalyst. The solid by-products, i.e., humins, were then valorized as a raw material for fabricating biochar-supported Lewis acid catalysts. The humins were collected and pretreated by AlCl3 as the impregnation agent, followed by carbonization. Detailed characterization revealed several Alsingle bondO species on the biochar surface plausibly in the amorphous state. The oxygen-containing functional groups of humins might serve as anchoring sites for the Al species during impregnation. The humins-derived biochars exhibited good catalytic activity toward glucose-to-fructose isomerization, a common biorefinery reaction catalyzed by Lewis acids. A fructose yield of up to 14 Cmol% could be achieved under microwave heating at 160 °C for 20 min in water as the greenest solvent. Such catalytic performance was comparable with the previously reported Al-based catalysts derived from wood waste and graphene/graphitic oxide. This study herein highlights humins as a low-cost alternative source of carbon for the preparation of renewable solid catalysts, proposing a novel practice for recycling by-products from food waste valorization to foster circular economy and sustainable development.